1. Field of the Invention
The present invention relates to a steering apparatus for vehicle provided with a so-called transmission ratio varying mechanism to permit alteration of a transmission ratio of turn angle of wheels to steering angle of a steering wheel.
2. Related Background Art
An example of the steering apparatus provided with such a transmission ratio varying mechanism is disclosed in Japanese Laid-open Patent Application No. Sho 63-227472, for example. As disclosed in this application, the transmission ratio varying mechanism is constructed in such structure that a prescribed gear mechanism connects an input shaft connected to the steering wheel side to an output shaft connected to the tie rod side and that this gear mechanism is driven by an actuator to permit alteration of the transmission ratio of rotations between the input shaft and the output shaft.
Control of driving of this actuator was executed by detecting a speed of the vehicle, a steering position of the steering wheel, and so on. When the steering wheel was manipulated at such a quick steering speed as to surpass response of the actuator, the actuator sometimes had a delay in follow-up. With occurrence of this delay in follow-up, even under such circumstances that a driver stopped manipulating the steering wheel after a quick steering operation, the actuator would operate by the degree corresponding to the follow-up delay to turn the wheels, and the driver could have an incompatible steering feeling.
The present invention has been accomplished in order to solve the above problem and an object thereof is to provide a steering apparatus for vehicle that can restrain the incompatible steering feeling caused by the follow-up delay of the actuator even under such circumstances of quick steering operation.
This steering apparatus for a vehicle comprising: (A) a transmission ratio varying mechanism capable of varying a transmission ratio of rotational angle of an output shaft to an input shaft connected to a steering handle; and (B) a steering control device that controls the transmission ratio, setting the transmission ratio by correcting an original transmission ratio when the difference between real and target rotational angles of the output shaft is greater than a predetermined value, the corrected transmission ratio being equal to or less than the original transmission ratio.
More precisely, the steering apparatus for vehicle is a steering apparatus for vehicle comprising a transmission ratio varying mechanism capable of varying a transmission ratio of turn angle to steering angle, the steering apparatus for vehicle comprising: steering angle detecting means for detecting a steering angle of a steering wheel; turn angle detecting means for detecting a turn angle of a wheel; transmission ratio setting means for setting the transmission ratio according to a running condition of vehicle; target turn angle setting means for setting a target turn angle of the wheel according to the steering angle detected by the steering angle detecting means, based on the transmission ratio set by the transmission ratio setting means; driving control means for controlling driving of the transmission ratio varying mechanism, based on a deviation between the turn angle of the wheel and the target turn angle; and compensating or correcting means for starting a compensation or correction process when the deviation is larger than a predetermined threshold value, the compensating means comprising target turn angle altering means for altering a value of the target turn angle so as to make the deviation smaller.
When the follow-up delay occurs in the transmission ratio varying mechanism because of the quick steering operation or the like, the deviation increases between the target turn angle and an actual turn angle. For example, when the driver stops manipulating the steering wheel under such circumstances, the transmission ratio varying mechanism operates by the degree commensurate with this deviation under control of the driving control means. When this deviation becomes greater than the predetermined threshold value, the apparatus then goes into the compensation process by the compensating means to alter the value of the target turn angle by the target turn angle altering means, thereby changing the deviation to a smaller value.
The compensating means further comprises transmission ratio altering means for altering the transmission ratio set by the transmission ratio setting means, based on a target turn angle resulting from alteration by the target turn angle altering means.
The transmission ratio set by the transmission ratio setting means is altered by the transmission ratio altering means, whereby during the compensation process the target turn angle setting means sets the target turn angle according to the steering angle, based on the transmission ratio after the alteration.
This steering apparatus comprises return means for terminating the compensation process of the compensating means, based on a comparison result between the transmission ratio set by the transmission ratio setting means and a transmission ratio resulting from alteration by the transmission ratio altering means.
When the comparison result between the transmission ratios is, for example, such that values of the two transmission ratios are nearly equal to each other or such that the magnitude relation between the two transmission ratios is inverted, the return means terminates the compensation process of the compensating means to return control to the normal control based on the transmission ratio set by the transmission ratio setting means.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.